1. Field of the Invention
The present invention relates to a fuel injection control device which controls a fuel injection to a diesel engine.
2. Description of the Related Art
An example of a conventional fuel injection control device is shown in U.S. Pat. No. 4,545,352 (corresponding to Japanese Unexamined Patent Publication No. 59-165858). In this conventional device, a fuel injector is provided with a needle valve which is positioned at a low position or a high position according to a pressure in a pressure control chamber, to selectively prevent or allow communication between a fuel chamber and a fuel injection aperture. The fuel chamber is always supplied with a pressurized fuel, and the pressure control chamber is pressurized or depressurized by an operation of a switching valve. That is, when the pressure control chamber is pressurized, the needle valve is lowered to prevent communication between the fuel chamber and the fuel injection aperture and stop a fuel injection, and when the pressure control chamber is depressurized, the needle valve is raised to allow communication between the fuel chamber and the fuel injection aperture to carry out a fuel injection.
To supply a constant high pressure fuel to the fuel chamber and the pressure control chamber, the conventional device is provided with a pump and a pressure regulator. However, it is technically difficult to obtain a constant high pressure fuel with a pump and a pressure regulator having a simple construction. Accordingly, in place of the pump and the pressure regulator, a simply constructed plunger mechanism in which a plunger having a spill port is slidably housed in a housing having a relief port is provided. In this plunger mechanism, the plunger moves forward to pressurize the fuel until the spill port communicates with the relief port so that the fuel is pressurized to a constant pressure. However, as the stroke of the plunger is always constant, in a low load condition in which a fuel injection period is short, the fuel is still pressurized after the fuel injection is finished. Such an excessive pressurization causes a power loss at the plunger mechanism, and in addition, fuel passages should not be subjected to a high pressure for a long time.